Household Secondary Attack Rate of COVID-19 and Associated Determinants

Abstract

Background: As of April 2, 2020, the global reported number of COVID-19 cases has crossed over 1 million with more than 55,000 deaths. The household transmissibility of SARS-CoV-2, the causative pathogen, remains elusive.

Methods: Based on a comprehensive contact-tracing dataset from Guangzhou, we estimated both the population-level effective reproductive number and individual-level secondary attack rate (SAR) in the household setting. We assessed age effects on transmissibility and the infectivity of COVID-19 cases during their incubation period.

Results: A total of 195 unrelated clusters with 212 primary cases, 137 nonprimary (secondary or tertiary) cases and 1938 uninfected close contacts were traced. We estimated the household SAR to be 13.8% (95% CI: 11.1-17.0%) if household contacts are defined as all close relatives and 19.3% (95% CI: 15.5-23.9%) if household contacts only include those at the same residential address as the cases, assuming a mean incubation period of 4 days and a maximum infectious period of 13 days. The odds of infection among children (<20 years old) was only 0.26 (95% CI: 0.13-0.54) times of that among the elderly (≥60 years old). There was no gender difference in the risk of infection. COVID-19 cases were at least as infectious during their incubation period as during their illness. On average, a COVID-19 case infected 0.48 (95% CI: 0.39-0.58) close contacts. Had isolation not been implemented, this number increases to 0.62 (95% CI: 0.51-0.75). The effective reproductive number in Guangzhou dropped from above 1 to below 0.5 in about 1 week.

Conclusion: SARS-CoV-2 is more transmissible in households than SARS-CoV and MERS-CoV, and the elderly ≥60 years old are the most vulnerable to household transmission. Case finding and isolation alone may be inadequate to contain the pandemic and need to be used in conjunction with heightened restriction of human movement as implemented in Guangzhou.

Figure 1.

Spatial distribution of COVID-19 case clusters based on contact-tracing data from Guangzhou through February 17, 2020. The upper-left panel shows the overall distribution in Guangzhou, and panels A-F show the distribution in the sub-regions defined in the upper-left panel. Cases are considered as primary (triangles) if their symptom onset dates are the earliest or one day after the earliest in the cluster and as non-primary (circles) otherwise. Infected household contacts are represented by dotted circles. Non-infected contacts are not shown. Cases imported from outside Guangzhou are colored in red and local cases are colored in blue. Darker (lighter) color indicates symptom onset on or before (after) Jan. 23, 2002. Each arrow indicates potential transmission direction from a case with an earlier symptom onset to an infected contact with a later symptom onset. Co-primary cases are linked by unidirectional dashed lines. The displayed location of each case is jittered away from the actual residential address. Population densities at the township level are shown as the background.

Figure 2.

Epidemic curve based on symptom onset dates of COVID-19 cases in Guangzhou from Jan. 6 to Feb. 18, 2020. Cases are stratified by imported vs. local and primary vs. secondary and shaded correspondingly. Here all non-primary cases are considered as secondary cases. Estimated effective reproductive numbers R_t are shown for three scenarios corresponding to assumptions about the transmission relationship between different types of cases (see methods, statistical analysis).